The present invention relates generally to feature imaging using scanning electron microscopy, and more specifically to apparatus and methods for enhancing image quality.
FIG. 1 is a diagrammatic representation of a conventional scanning electron microscopy configuration 100. As shown, a beam of electrons 102 is scanned over a specimen 104 (e.g., a semiconductor wafer). Multiple raster scans 112 are typically performed over a small area 114 of the specimen 104. The beam of electrons 102 either interact with the specimen and cause an emission of secondary electrons 106 or bounce off the specimen as backscattered electrons 106. The secondary electrons and/or backscattered electrons 106 are then detected by a detector 108 that is coupled with a computer system 110. The computer system 110 generates an image that is stored and/or displayed on the computer system 110.
Although conventional microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the specimen under certain conditions. For example, conventional systems and techniques fail to produce images for certain specimen features. For example, the bottom of a contact or trench region is typically undifferentiated from the adjacent sidewalls of the contact or trench. Typically, both the bottom and sidewalls will appear dark because a significant number of the secondary electrons within the contact or trench hit the sidewalls and fail to escape from the contact or trench and reach the detector 108. As a result of this failure, the bottom and sidewall""s individual dimensions and shapes are obscured within the resulting image.
Improved conventional systems provide multi-perspective imaging (e.g., of sidewalls and trenches) by providing multiple angles between the incident beam and the specimen. The multiple angles are typically provided by either tilting the stage on which the specimen is positioned relative to the electron beam or by tilting the electron beam column through which the incident electron beam is generated and directed towards the specimen relative to the specimen. In the first configuration, a first image is obtained from the sample when the specimen and stage are positioned at a first angle relative to the beam column, and a second image is obtained when the specimen and stage are positioned at a second angle relative to the beam column In the second configuration, a first image is obtained from the sample when the beam column is positioned at a first angle relative to the specimen, and a second image is obtained when the beam column is positioned at a second angle relative to the specimen.
Although these two configurations for obtaining multi-perspective imaging are adequate, they both have associated disadvantages. Specifically, both configurations require a high number of moving parts. That is, the first configuration requires movement mechanisms for accurately positioning the stage angle, while the second configuration requires movement mechanisms for accurately positioning the beam column angle. The movement mechanisms in either configuration are typically complex and expensive. Additionally, both configurations require constant monitoring and adjustment to ensure proper beam alignment relative to the specimen area of interest or an accurate stage position relative to the beam column.
Thus, improved microscopy apparatus and techniques for accurately and efficiently providing multiple perspective images are needed.
Method and apparatus for imaging at multiple perspectives of a specimen are disclosed. In one embodiment, an apparatus for generating a multi-perspective image using multiple charged particle beams (e.g., electron beams) directed at a specimen at multiple incident angles is disclosed. The apparatus generally includes a charged particle beam generator system arranged to generate and control a first charged particle beam directed substantially at a first angle towards the specimen and a second charged particle beam directed substantially at a second angle towards the specimen. The apparatus also includes an image generator arranged to generate one or more images based on charged particles emitted from the specimen in response to the first and second charged particle beams and a controller arranged to cause the charged particle beam generator to direct both the first charged particle beam and the second charged particle beam at a first area of the specimen. In a specific implementation, the charged particles are in the form of electrons and the apparatus is a dual electron beam scanning electron microscope (SEM).
In a preferred aspect, the controller is only operable to move the stage in a translational direction with respect to a surface of the specimen. In a specific embodiment, the first angle is about 90 degrees and the second angle is about 45 degrees. In another embodiment, the first angle is about 90 degrees and the second angle is about 30 degrees.
In a specific aspect, the controller is further operable to cause the charged particle beam generator system to direct the first charged particle beam towards the first area at a first time and to direct the second charged particle beam towards the first area at a second time that differs from the first time. In a further aspect, the apparatus also includes a first detector for detecting charged particles from the specimen in response to the first charged particle beam being directed towards the specimen and generating a first detected signal and a second detector for detecting charged particles from the specimen in response to the second charged particle beam being directed towards the specimen and generating a second detected signal. In this aspect, the image generator is operable to receive the first and second detected signals and use them to detect defects in the specimen.
In a specific implementation, the charged particle beam system is in the form of a first charged particle beam generator for directing the first charged particle beam towards the specimen and a second charged particle beam generator for directing the second charged particle beam towards the specimen. In a further aspect, the first and second charged particle beam generators are positioned to direct charged particle beams onto different areas of the specimen. The apparatus may further include a multiplexer coupled between the controller and the first and second charged particle beam generators, and the first and second charged particle beam generators are controlled independently by the controller through the multiplexer such that the first charged particle beam is directed towards a first area at a first time period and the second charged particle beam is directed towards the same first area at a second time period that differs from the first time period. The controller is further operable to move the specimen such that the first area is under the first charged particle beam during the first time period and under the second charged particle beam during the second time period.
In an alternative implementation, the charged particle beam system is in the form of a single beam generator for generating a single incident beam and a beam splitter for receiving and splitting the single incident beam into the first charged particle beam and the second charged particle beam and directing the first charged particle beam at the first angle and the second charged particle beam at the second angle towards the specimen.
In yet another implementation, the charged particle beam system is in the form of a first charged particle beam generator for generating the first charged particle beam towards the specimen, a second charged particle beam generator for generating the second charged particle beam towards the specimen, and a lens for receiving the first and second charged particle beams and directing them towards the specimen at a single area In one aspect, the first and second charged particle beam generators are integrated together along with the lens.
In an alternative embodiment, the invention pertains to a method of generating an image of a specimen with a scanning electron microscope (SEM) having an electron beam generator for directing a first electron beam substantially towards the specimen at a first angle relative to a surface of the specimen and for directing a second electron beam substantially towards the specimen at a second angle relative to a surface of the specimen, at least one detector for detecting electrons that are emitted from the specimen, and an image generator for generating the image of the specimen from the emitted electrons. A first incident electron beam is directed towards a first area of the specimen at the first angle. Without tilting the specimen relative to the beam generator, a second incident electron beam is directed towards the first area of the specimen at the second angle. The first angle differs from the second angle. One or more images are then generated based on electrons detected by the at least one detector in response the first and second incident beams.
In one specific implementation, the first incident beam and the second incident beam are directed towards the first area during different time periods. In a further aspect, the specimen is moved relative to the beam generator such that the first area is below the first incident beam during the first time period and the first area is below the second incident beam during the second time period. That is, the specimen or the beam generator is moved. In one example, the specimen is moved translationally. In another aspect, the first incident beam and the second incident beam are directed towards the first area during a same time period.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.